1. Technical Field
The present invention relates to radio-frequency phase shifters and more particularly to phase shifters operating at millimeter-wave frequencies for integrated phased arrays systems.
2. Description of the Related Art
Phase sifters and phased arrays are now presented in a context which illustrates their requirements for monolithic integration and the existing implementations. Phased Array Systems: Phased array transceivers are a class of multiple antenna systems that achieve spatial selectivity through control of the time delay differences between successive antenna signal paths. A change in this delay difference modifies the direction in which the transmitted/received signals add coherently, thus “steering” the electromagnetic beam. The integration of phased-arrays in silicon-based technologies has aroused great interest in recent times due to potential applications in high-speed wireless communication systems and radar.
There are several prominent commercial applications of phased arrays at millimeter-wave frequencies. The 7 GHz Industrial, Scientific and Medical (ISM) band at 60 GHz is currently being widely investigated for indoor, multi-gigabit per second Wireless Personal Area Networks (WPANs). In such an application, the line-of-sight link between the transmitter and receiver can easily be broken due to obstacles in the path. Phased arrays can harness reflections off the walls due to their beam-steering capability, thus allowing the link to be restored.
Referring to FIG. 1A, a block diagram illustrates a 1-D N-element phased array receiver 10, with an inter-element antenna spacing of d=λ/2, where λ is the free-space wavelength corresponding to the frequency of operation, ω. When a signal 12 of amplitude A from an electromagnetic beam is incident to the array 10 at an angle θin (measured from the normal direction), the electromagnetic wave experiences a time delay in reaching the successive antennas 16 from the phase gap indicated by dsin(θin). Variable time delay blocks 14 in each signal path in the receiver compensate for this propagation delay. In this way, with appropriate delays at each element, where the outputs of the elements are labeled S1(t) through SN(t), the combined or summed output signal Scomb(t) from summer 18 will have a larger amplitude than it could be obtain with a single element. The phased array factor (AF), in the context of receivers, is defined as the additional power gain achieved by the array over a single-element receiver.
The phased array factor is a function of the angle of incidence (θ) and the array's progressive delay difference (τ), and hence reflects the spatial selectivity of the array. The beam-pointing direction θm is the incident angle corresponding to maximum power gain. FIG. 1B shows an array factor of a 4-element phased array for different Δτ settings, resulting in different beam-pointing directions. The curves measure the response of the four different elements in the array.